Method of accelerating ions



Nov. 27, 1951 E. E. HAYS METHOD OF ACCELERATING IONS Filed Oct. 6, 1949 FIG. I

HIGH VOLTAGE PULSER ION COLLECTOR I8 OUTER IIZ'HELICAL WINDING nmmccmcmmcmcmn I w s T0 VACUUM PUMP FIG.3.

INNER HELICAL WINDING /0UTER HELICAL WINDING INNER HELICAL WINDING INVENTOR. EARL E. HAYS Patented Nov. 27, 1951 METHOD OF ACCELERATING IONS Earl E. Hays, Upton, N. Y., assignor to the United States of America as represented by the United States Atomic Energy Commission Application October 6, 1949, Serial No. 119,968

Claims.

The present invention relates to a method of accelerating ions.

The present invention has particular utility in connection with the tinre-of-fiight mass spectrometer disclosed in the copending application of S. A. Goudsmit, S. N. 83,258, filed March 24, 1949. Ion sources are usually operated in conjunction with mass spectrometers which preferably employ a steady stream of ions. The Goudsmit spectrometer measures the time-of-fiight of ions from a source to a collecting plate. In this connection an interrupted flow of ions is necessary since the measurement is made from the time a pulse of ions leaves the source until the time when it arrives at the collector plate. It is desirable in operating a mass spectrometer of this type to have a large number of pulses of ions leave a source and arrive at a plate in order that use may be made of an oscillograph device to measure the time-of-fiight of the ions.

It is accordingly an object of the present invention to provide a method of operating an ion source which produces a large number of ion pulses.

It is another object of the-present invention to provide a method of operating an ion source which accelerates ions to uniform momenta.

It is a further object of the present invention to provide a method of operation of an ion source which produces a large number of ions which travel in a uniform direction from the source.

It is still a further object of the present invention to provide a method of operating an ion source which causes ions of different weights to describe the same path through a magnetostatic field.

Other objects and advantages will be in part obvious and in part pointed out hereinafter.

In general, the objects of the present invention may be achieved by accelerating ions in a substantially uniform direction and to substantially uniform momenta by generating ions in a magnetic field, and accelerating these ions through an electric field for a relatively small fraction of the time necessary to eject them from the electric field.

The advantages of the present invention may be best pointed out by reference to the specific illustration of the method of operating the source in conjunction with the time-of-fiight mass spectrometer of Goudsmit referred to above although it will be understood that the invention is not limited to operation in this connection but the method of the present invention may be applied to accelerating ions to uniform velocities and in uniform direction for any apparatus where ions having these characteristics are desired.

Reference is made in the description to the accompanying drawing wherein:

Figure l is a diagrammatic view of an ion source assembly adapted to be mounted in the analyzer chamber of a time-of-fiight mass spectrometer.

Figure 2 is a diagrammatic view partly in section, and on a reduced scale, showing a preferred arrangement of the ion source assembly, the ion collector and the analyzer chamber of a mass spectrometer suitable for carrying out the present invention.

Figure 3 is a horizontal section of the chamber taken along a line 3--3 of Figure 2.

Referring first to Figure 1 there is illustrated generally the essential parts of an ion source arrangement, indicated as II6 on the smaller scale Figures 2 and 3. As will later appear, ions generated thereby traverse a magnetic field substantially normal to their path and in order to carry out the improved method of the present invention such ion source must be enclosed in an evacuated analyzer chamber or tube I I 0, such as that illustrated in Figure 2. Conventional mass spectrometer chambers in cylindrical tank form may of course also be employed. For the purpose of forming the ions, which will be bunched or pulsed as later described, a metal plate I00 is provided attached to one end of a ceramic container I04. The plate I00 may be heated by passing a current through resistance heater coil I02 enclosed within the container I04. Voltage may be applied to the coil I02 from the source I03 to cause a current to flow through the coil. A grid I08, which is preferably disposed parallel to the surface of the plate I00, is spaced at a distance from the plate at a point along the line normal to the surface of the plate I00. The plate I00 and grid I08 are electrically connected to an intermittent source of high voltage I 05 through the conductors 91 and 99 respectively.

The ion source arrangement, designated II6 such as that described above when operated according to the method of the present invention may be positioned in an evacuated container IIO enclosing a generally annular chamber such as that illustrated in Figures 2 and 3. Two sets of electrical windings H2 and H4 wound respectively about the external and internal surfaces of the container H0 and connected to the conventional power sources H3 and H5 respectively establish a magnetostatic field within the container when a constant current is passed through. the windings. The lines of flux are generally parallel to the central axis I20 of the container and the ion source arrangement accordingly is mounted within the evacuated container or analyzer tube so that the parallel plate I and grid I08 are properly located with respect to said lines of flux whereby a curvilinear movement" of the ions results from the coordinated action of the electric and magnetic fields, analogous to that employed in conventional miass spectrometers, cyclotrons and the like. For reasons which will be pointed out hereinafter ions which are emitted from the source I It may be made to describe generally helical paths within the chamber so as to arrive at the collector H8.

Referring again to Figure l, in operation a stream of a gas which may be ionized by thermal contact with the heated metal plate I00 is directed against this plate along the line indicated by the arrow 105' from a gas source; One such source. is described inv the copending application of Goudsmit referred to above. The use of. other sources to produce the same effect is of course contemplated. For example, a fine bore'gas conduit 101' may supply such a gas from a source (not shown) external to the evacuated chamber. It hasbeerrifoundthat most of the ions which are formed remain in contact with or very close to the surfaceof the plate Hill. The plate I 00 isikept heatedby' the passage of current from. the'source I03. through the resistance coil I02. A voltage is applied intermittently between the grid H18 and the'plate l Bil so'as-to urge the'ions formed at the plate 1 90 to move toward the grid I08. According to the preferred procedure this voltage is applied for suflicient time only to cause the ions to move a-small fraction of the distance between the plate H32} and. the grid I98. Since most of the ions which are: formed are located at the plate I03 when the voltage is applied and sincethey travel butv a short distance before the voltage is shut off, by the time they reachzthe grid I08 it is essential- 1y: uncharged and the curvilineal movement of the ions through the openings in the gird is not disturbed by electrical atraction toward the grid strands. For this reason, once the ions have-been accelerated according to the present method they continue to follow the paths along which they are accelerated in traveling through an instrument such as the" time-of-flight spectrometer; Further,. since rnpst of the ions are at a maximum distance from the grid at the start of their'accelera ting period (remaining close to or in contactwithplate I 50) they'are accelerated in a substantially uniform direction which, in the illustrati'on" given, is substantially normal tothe'plate Hliland to" the grid 1 08. The combined effect of these phenomena is the acceleration of most of the ions at the source when the voltage is applied, along substantially uniform paths of travel; The scattering of the ions due to attractiorr toward the grid strands: in passing through a charged: grid is eliminated and a high efiiciency of operation is achieved; By inclining the source axis AA slightly upward with respect to the horizontal the accelerated ions may be made to describe helical paths in their travel through a magnetic field, the lines of flux of which are yertically disposed. The source should preferably be positioned in a magnetic field with the lines of flux; approximately parallel to the face of plate I00 so that the electrons and ions dissociated by the heat of the plate will not recombine to form atoms-"and moiecules of'gas. The recombination is inhibited by the magnetic field because in crossing lines of flux thereof the electrons and positively charged ions are caused to curve in opposite directions thereby tending to keep them separated and prevent recombination.

When the source is used in conjunction with a magnetostatic field the paths of travel of the ions have the same radius of curvature. This results from the fact that the curving action of'the field on the ions depends on their momenta and substantially all the ions accelerated according to the. present method have the same momentum. Uniform momenta are achieved because very few ions: leave. the electric field before the voltage is shut off and consquently substantially all the ions aresubjected to the same electric field for the same. length of. time. Thus, having the same momentum and the same initial direction of travel, the ion paths are confined to an annular volume such as that enclosed by the container illustrated in-EigureZ.

This procedure is distinct from conventional methods. By conventional-methods the voltage is applied from. the time the ion is formed until it has been accelerated from the electric field. The lighter ions which travel faster therefore leave the source first with less momentum than the. heavier ions which travel slower and are sub-- 3' ectedtothe force of the field for a longer time;

By suitable choice of distances and voltages, ionshaving. desired momenta and consequently desired paths may be accelerated according to the'present mf-ethod.

The break-down voltage between opposed electrically conductive surfaces constitutes the upper limit of the voltage which. may be applied between the plate and grid. This. breah-downvoltage. is-common knowledge obtainable from. standard reference books. Generally lower voltages result inthe production of lower ion currents but no lower limit existsuntil the ion current disappears. In a particular apparatus where the-separation-between. the. grid andplate is. in the order of a centimeter and where a very high vacuum in the order of 10- millimeters of mercury is. established; avoltageinthe order of approximately 1-000 volts. is. preferred although voltages. ash-igh as 5000- volts' are practical. In general, using higher voltages results in the production of higher ion cur-rents. The spacing between the plate and grid must be adjusted in conformity with the voltage applied to produce ions of. desired momenta.v Greater separation between: grid and platenecessitates application of higher voltages for the samie time to produce accelerated; particles having the same momentum. Alternatively, the greater separation between the plate and gridwill be overcome by a longer application of the same accelerating voltage tl'ierebetween' to produce ions having the same momentum. Spacings of between one-half centimeter to two centimeters are preferred in a high vacuum model of the Goudsmit spectrometer referred to above. However, separations as small as one millimeter and as high as one meter aresuitable.- There is no upper limit on the separation of the'plate and grid if a suificiently high voltage is applied therebetweenexcept the practical limitation of the size of the'apparatus which must be operated at very low pressure. The fraction of the distance between plate and gridthrough which the ions may be accelerated before distortion of the curvilineari-ty' of the path of travel. is brought about is approximately two-thirds of the distance there- 'between; No lower limit exists sincea very slight motion of the particles through an extremely small distance will be sufiicient to start their travel through the spectrometer. With regard to the time interval between the application of voltage pulses it is essential that the ions pass through the grid I08 before the succeeding voltage application is made. It is preferred to apply the accelerating voltage for sufiicient time to accelerate the ions through less than of the distance between the charged elements but in general for not less than one one-hundredth of a microsecond. It has been found possible in operating such a source according to the present method to achieve a suitable spacing of voltage pulses and to accelerate the ions through the desired distance by applying a voltage for approximately of a microsecond at intervals of one millisecond where the clearance between the plate and grid was in the order of one centimeter, a voltage of approximately 150 volts was applied and the magnetostatic field maintained at about 600 gauss. Employing these characteristics rubidium ions take approximately 77 seconds in passing from the source to the collector through this field and describe a path having a diameter of about 8 inches.

It may be seen from the foregoing that the present invention provides a very effective method for accelerating ions having the same momentum and consequently having substantially uniform paths of travel through a magnetostatic field. This latter phenomenon makes the method of the present invention particularly advantageous in connection with the time-of-fiight mass spectrometer or similar device because any irregularities in the magnetostatic field such as slight deviations in uniformity will not afiect the accuracy of the instrument because all of the ions will be deviated in the same manner through the non-uniform region because of their identical momentum. A further advantage resides in the types of magnetic fields with which the present method can be employed. For example, referring to Figure 2, a generally ring-shaped field can be formed within the evacuated ring-shaped container I I by disposing electrical windings I2 and H4 about the exterior and interior surfaces of the ring chamber. Ions accelerated from a source H6 such as that illustrated in Figure 1 may be given just sufficient momentum, to make them describe a helical path having a radius equal to that of the ring container H0. Their helical paths of flight will carry them to the detecting instrument I I8 which may be of the conventional ion collector type such as that referred to in the copending Goudsmit application. Conventional methods of accelerating ions would not be feasible in conjunction with an evacuated chamber such as that illustrated in Figure 2 since the paths of travel of the ions having widely divergent momenta would result in a large number of such ions striking the walls of the container H0. As a further advantage it is possible employing the present method to produce timing time-of-fiight or other similar characteristics of the accelerated ions.

Since many embodiments might be made of the present invention and. since many changes might be made in the embodiment described, it is to be understod that the foregoing description is to be interpreted as illustrative only and not in a limiting sense.

I claim:

1. A method of accelerating ions which comprises generating said ions in a magnetic field and subjecting said ions to the accelerating effect of an electric field for a fraction of the time necessary to remove them from said electric field.

2. A method of accelerating ions which comprises generating said ions in a magnetostatic field and subjecting said ions to the accelerating action of an electric field for less than two-thirds of the time necessary to eject them from said electric field.

3. A method of accelerating ions which comprises generating said ions in a magnetic field and subjecting said ions to the accelerating action of an electric field of between and 5,000 volts per centimeter for less than two-thirds the time necessary to eject them from said electrostatic field.

4. A method of accelerating ions which comprises generating said ions in a magnetic field and subjecting said ions to the accelerating action of an electric field for less than two-thirds of the time necessary to eject them from said electric field, said magnetic field being substantially perpendicular to said electric field.

5. A method of accelerating ions which comprises generating said ions in a magnetic field and subjecting said ions to the accelerating action of an electric field for a time greater than one one-hundredth of a microsecond but for less than two-thirds the time necessary to remove said ions from said electric field.

EARL E. HAYS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,140,284 Fornsworth Dec. 13, 1938 2,241,976 Blewett et a1. May 13, 1941 2,331,190 I-Iipple Oct. 5, 1943 2,331,788 Baldwin Oct. 12, 1943 2,397,560 Olesen Apr. 2, 1946 2,457,162 Langmuir Dec. 28, 1948 2,465,786 Blewett Mar. 29, 1949 OTHER REFERENCES Atomic Energy Commission AECD2117, High Voltage Pulser for 184 inch Cyclotron Electric Deflector, Kerns, Baker, Edwards, Farly, April 24, 1948.

Bulletin of the American Physical Society, vol.

marks on an oscillograph signal to measure the 5 21. N 2, April 25. 1946. P 2; S e 1- 

